Canopy Gradients in Fluxes of Carbonyl Sulfide and CO2 in a Tropical Rainforest

Abstract:

Carbonyl sulfide (COS) is a promising tracer in terrestrial carbon cycle research because the atmospheric drawdown of COS is dominated by vegetation uptake. Concurrent measurements of atmospheric COS and CO2 have been proposed to quantify the terrestrial gross fluxes of photosynthesis and respiration at ecosystem to large scales. The application of COS as a carbon cycle tracer is based on the close coupling of the uptake of COS and CO2 during canopy photosynthesis. Although tropical forests are responsible for about a quarter of global photosynthesis, our understanding of canopy processes in tropical forests is limited by lack of field data. For COS and CO2 fluxes, this is particularly critical since the COS:CO2 leaf relative uptake (LRU) is affected by light, and tropical canopies are typically characterized by strong vertical light gradients. Here, we report field data on leaf COS and CO2 fluxes and LRU from a tropical rainforest in Costa Rica, covering the end of the rainy period in late 2013 and start of a (relatively) drier period in early 2014. We collected continuous data on three dominant tree species using branch chambers located in the sunny top layer, partly-sunny upper intermediate layer, and permanently shaded lower part of the canopy. Both COS and CO2 uptake fluxes were consistently largest in the partly-sunny intermediate canopy. The deeply shaded lower canopy had lower daytime fluxes, but continued to take up COS at night, indicating that these leaves are in a more sheltered environment resulting in incomplete stomatal closure at night. We observed strong vertical gradients in LRU with depth in the canopy, predominantly related to the changing light environment. We then used fluxes and light gradients interpolated from leaf-scale observations to obtain whole-canopy COS and CO2 fluxes and LRU. Our results highlight that the contributions of the lower, shaded portions of the canopy will need to be taken into consideration for COS-based ecosystem flux partitioning as well as regional to global scale photosynthesis estimates from atmospheric COS measurements.